Supply to Summer House

Hi all

I've agreed to help a relative supply power to his summer house and wanted to run my plan by you and ask for any advice you might have. I'll give you as much detail as possible - some might even be irrelevant.

-My uncle requires three double sockets in the summer house with a view to running two lamps, a laptop and printer, and on occasions a 2kw plug in heater

-The summer house is located 15metres from the back of the house

-The house is terraced meaning any supply will have to run through the house however the flooring is all parquet which my uncle is reluctant to lift

-The house currently has no RCD protection

-My plan is to spur from a socket in the back room to a 13A RCD Fused Connection Unit, go through the wall to a junction box, run 3core SWA cable (using 3rd core as a CPC) along a pre installed conduit that runs underground from the back of the house to the summer house, terminate the swa in a junction box inside the summer house, run twin and earth out to the sockets in series.

My main questions are:

-What size cabling should I use? (I figured 4mm SWA and 2.5mm T&E but wasn't sure about factors such as volt drop)

-Do I need any additional circuit protection inside the summer house?

-Is it acceptable to use the 3rd core of the SWA as a cpc rather than the armour?

-Seeing as taking the supply directly from the consmer unit has been ruled out, are there any better/safer/easier options to spurring?

Any advice you can offer would be greatly appreciated - thanks

AD23 said:

Any advice you can offer would be greatly appreciated - thanks

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Please use an electrician for this work.

Believe me, I've told him that's his best bet. He's adament that he wants to do it himself though. I just want to make sure that he does it safely.

The work described is notifiable to building control, therefore if doing it yourself a fee will be payable before any work is started. Typically this will be £200 upwards, and can be a lot more in some areas.
To find out which local authority and therefore how much they charge, enter the postcode of the property here: http://www.labc.uk.com/

To ensure it is safe, it will need to be designed and installed correctly, and then tested before any power is connected.

Forgetting about Part P here is the problems.

1) You don't know the loop impedance at the point you want to tap into so you can't work out what size cable is required to ensure the loop impedance stays within limits.
OK using a RCD FCU will remove some of the problems but to keep within the 3% volt drop for lights you will need the line - neutral impedance or prospective short circuit current to work out if your within the limits.

2) You propose to draw over the 2kW limit for any fixed appliance supplied from a final ring. So to comply with BS7671:2008 you will need a dedicated supply.

With this in mind clearly even if you intend to notify the LABC your plans are flawed.

So options are:-
1) Ditch the heater.
2) Run back to consumer unit.

If you ditch heater then you may be able to use thinner cable depending on the loop readings.

So post the loop impedance or prospective short circuit readings and then some one may be able to help. But will need those readings.

eric: heater is not more than 2kW.

3% VD isnt an issue either as he wants to use table lamps by the sound of it.

Heater, lamps and laptop are over 2kW and one must consider the whole Summer house as an appliance so I suppose one could fit a 7A fuse in the FSU and then it would comply?

But without measurements can't see how anyone can advise on cable size so as to if over or under 2kW is really minor the question is what is the loop impedance at the point where the FCU is to be fitted then one can start working out what is required.

Somehow DIY people seem to think we buy our tools for fun and they are not required!!!

AD23 said:

Believe me, I've told him that's his best bet. He's adament that he wants to do it himself though. I just want to make sure that he does it safely.

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• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm (all subsections)
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• //www.diynot.com/wiki/electrics:mcb2
  
  
• http://www.tlc-direct.co.uk/Book/7.13.3.htm
  
  
• http://www.tlc-direct.co.uk/Book/8.1.1.htm
  
  
• //www.diynot.com/wiki/electrics:swa-gland
  
  
• http://electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf
  
  
• //www.diynot.com/wiki/electrics:part-p
  

I bet that if your installation has no rcd protection that it is probably not segregated into enough circuits for you to add this extra load to the, or one of the 2 socket circuits anyway..

You need an electrician for this. I know this is a diyforum but somethings are better left to experts.

Its free to get 3 quotes.